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Chapter 22 Carbohydrates

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Chapter 22 Carbohydrates Introduction Classification of Carbohydrates Carbohydrates have the general formula C x(H 2O) y Carbohydrates are defined as polyhydroxy aldehydes or ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates Disaccharides can be hydrolyzed to two monosaccharides Oligosaccharides yield 2 to 10 monosaccharides Polysacccharides yield >10 monosaccharides Chapter 22 2 Photosynthesis and Carbohydrate Metabolism Carbohydrates are synthesized in plants by photosynthesis Light from the sun is absorbed by chlorophyll and this is converted to the energy necessary to biosynthesize carbohydrates Carbohydrates act as a repository of solar energy The energy is released when animals or plants metabolize carbohydrates Much of the energy released by oxidation of glucose is trapped in the molecule adenosine triphosphate (ATP) The phosphoric anhydride bond formed when adenosine triphosphate (ADP) is phosphorylated to make ATP is the repository of this energy This chemical energy is released when ATP is hydrolyzed or a new anhydride linkage is created Chapter 22 3 Chapter 22 4 Monosaccharides Classification of Monosaccharides Monosaccharides are classified according to: (1) The number of carbon atoms present in the molecule and (2) whether they contain an aldehyde or ketone group D and L Designations of Monosaccharides The simplest carbohydrates are glyceraldehyde, which is chiral, and dihydroxyacetone, which is achiral Glyceraldehyde exists as two enantiomers Chapter 22 5 In the early 20th century (+)-glyceraldehyde was given the stereochemical designation (D) and (-)-glyceraldehyde was given the designation (L) A monosaccharide whose highest numbered stereogenic center has the same configuration as D -(+) -glyceraldehyde is a D sugar A monosaccharide whose highest numbered stereogenic center has the same configuration as L-(-)-glyceraldehyde is an L sugar Chapter 22 6 Structural Formulas for Monosaccharides Fischer projections are used to represent stereochemistry in carbohydrates In Fischer projections horizontal lines are understood to project out of the plane toward the reader and vertical lines are understood to project behind the plane A Fischer projection cannot be removed from the plane of the paper or turned 90 o and still represent the molecule accurately Glucose exists primarily in two cyclic hemiacetal forms that are diastereomers of each other The cyclic hemiacetal forms interconvert via the open -chain form The cyclic hemiacetals differ only in configuration at C1 and are called anomers The carbon at which their configurations differ is called the anomeric carbon The ααα-anomer has the C1 hydroxyl trans to the -CH 2OH group The βββ-anomer has the C1 hydroxyl cis to the -CH 2OH group The flat cyclic representation of carbohydrates is called a Haworth formula Cyclic glucose actually exists in the chair form Chapter 22 7 Chapter 22 8 In the βββ-anomer of glucose all groups around the ring are equatorial The configuration at the anomeric carbon of a cyclic carbohydrate can be left unspecified using a wavy bond Fischer projections and Haworth formulas can easily be interconverted (next slide) A six-membered ring monosaccharide is designated a pyranose, e.g ., glucose in this form is glucopyranose A five-membered ring monosaccharide is designated a furanose, Chapter 22 9 Chapter 22 10 Mutarotation The ααα- and βββ-forms of glucose can be isolated separately Pure ααα-glucose has a specific rotation of +112 o Pure βββ-glucose has a specific rotation of +18.7o When either form of glucose is allowed to stand in aqueous solution, the specific rotation of the solution slowly changes to +52.7o It does not matter whether one starts with pure ααα- or βββ-glucose Mutarotation is the change in optical rotation as an equilibrium mixture of anomers forms Mutarotation of glucose results in an equilibrium mixture of 36% ααα-glucose and 64% βββ-glucose The more stable βββ-glucose form predominates A very small amount of the open-chain form exists in this equilibrium Chapter 22 11 Chapter 22 12 Glycoside Formation Glycosides are acetals at the anomeric carbon of carbohydrates When glucose reacts with methanol in the presence of catalytic acid, the methyl glycoside is obtained A glycoside made from glucose is called a glucoside Chapter 22 13 Chapter 22 14 Glycosides can be hydrolyzed in aqueous acid The alcohol obtained after hydrolysis of a glycoside is called an aglycone Chapter 22 15 Other Reactions of Monosaccharides Enolization, Tautomerization and Isomerization Monosaccharides dissolved in aqueous base will isomerize by a series of keto-enol tautomerizations A solution of D-glucose containing calcium hydroxide will form several products, including D-fructose and D-mannose A monosaccharide can be protected from keto-enol tautomerization by conversion to a glycoside Chapter 22 16 Formation of Ethers The hydroxyl groups of carbohydrates can be converted to ethers by the Williamson ether synthesis Benzyl ethers are commonly used to protect or block carbohydrate hydroxyl groups Benzyl ethers can be easily removed by hydrogenolysis Exhaustive methylation of methyl glucoside can be carried out using dimethylsulfate Chapter 22 17 Exhaustive methylation can be used to prove that glucose exists in the pyranose (6-membered ring) form Hydrolysis of the pentamethyl derivative results in a free C5 hydroxyl Silyl ethers are also used as protecting groups for the hydroxyls of carbohydrates Sterically bulky tert -butyldiphenylsilyl chloride (TBDPSCl) reacts selectively at the C6 primary hydroxyl of glucose Chapter 22 18 Conversion to Esters Carbohydrates react with acetic anhydride in the presence of weak base to convert all hydroxyl groups to acetate esters Conversion to Cyclic Acetals Carbohydrates form cyclic acetals with acetone selectively between cis-vicinal hydroxyl groups Cyclic acetals from reaction with acetone are called acetonides Chapter 22 19 Oxidation Reactions of Monosaccharides Benedict’s or Tollens’ Reagents: Reducing Sugars Aldoses and ketoses give positive tests when treated with Tollens’ solution or Benedict’s reagent + 0 Tollens’ reagent [Ag(NH 3)2OH] gives a silver mirror when Ag is reduced to Ag Benedict’s reagent (an alkaline solution of cupric citrate complex) gives a brick red precipitate of Cu 2O In basic solution a ketose can be converted to an aldose that can then react with Tollens’ or Benedicts’s reagent Chapter 22 20 Carbohydrates with hemiacetal linkages are reducing sugars because they react with Tollens’ and Benedict’s reagents The hemiacetal form is in equilibrium with a small amount of the aldehyde or ketone form, which can react with Tollens’ and Benedict’s reagents Carbohydrates with only acetal groups (glycosidic linkages) do not react with these reagents and are called non-reducing sugars Acetals are not in equilibrium with the aldehyde or ketone and so cannot react with these reagents Chapter 22 21 Bromine Water: The Synthesis of Aldonic Acids Bromine in water selectively oxidizes the aldehyde group of an aldose to the corresponding carboxylic acid An aldose becomes an aldonic acid Nitric Acid Oxidation: Aldaric Acids Dilute nitric acid oxidizes both the aldehyde and primary hydroxyl groups of an aldose to an aldaric acid Chapter 22 22 Periodate Oxidations: Oxidative Cleavage of Polyhydroxy Compounds Compounds with hydroxyl groups on adjacent carbons undergo cleavage of carbon-carbon bonds between the hydroxyl groups The products are aldehydes, ketones or carboxylic acids With three or more contiguous hydroxyl groups, the internal carbons become formic acid Cleavage also takes place when a hydroxyl group is adjacent to an aldehyde or ketone group An aldehyde is oxidized to formic acid; a ketone is oxidized to carbon dioxide Chapter 22 23 No cleavage results if there are intervening carbons that do not bear hydroxyl or carbonyl groups Chapter 22 24 Reduction of Monosaccharides: Alditols Aldoses and ketoses can be reduced to alditols Chapter 22 25 Reactions of Monosaccharides with Phenylhydrazine: Osazones An aldose or ketose will react with three equivalents of phenylhydrazine to yield a phenylosazone The mechanism for phenylosazone formation involves the following steps: Chapter 22 26 Reaction of D-Glucose or D-Mannose with excess phenylhydrazine yields the same phenylosazone Therefore, D-glucose and D-mannose differ in configuration only at C2 Compounds that differ in configuration at only one stereogenic center are called epimers Chapter 22 27 Synthesis and Degradation of Monosaccharides Kiliani-Fischer Synthesis The carbon chain of an aldose can be extended by: Addition of cyanide to epimeric cyanohydrins Hydrolysis to a mixture of epimeric aldonic acids or aldonolactones Reduction of the epimeric aldonic acids or aldonolactones to the corresponding aldoses For example, D-glyceraldehyde can be converted to D-erythrose and D-threose by the Kiliani-Fischer synthesis Chapter 22 28 Chapter 22 29 The Ruff Degradation An aldose can be shortened by one carbon via: Oxidation with bromine water to the aldonic acid Oxidative decarboxylation with hydrogen peroxide and ferric sulfate Chapter 22 30 The D Family of Aldoses Most biologically important aldoses are of the D family Chapter 22 31 Disaccharides Sucrose (Table sugar) Sucrose is a disaccharide formed from D-glucose and D-fructose The glycosidic linkage is between C1
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